Disappearing 3G icon and Wifi issue?

I have some issue with my Wi-fi. At school, I can connect to the wireless network with the app Wi-fi finder however, randomly it would disconnect me even when I am the same area with strong wireless connection.

Also, sometimes, I run into the issue where when it disconnects from the Wi-fi, the 3g icon won't return which returns in me having no data when trying to browser the web.